There is a "before" shot in that blog post as well, but the short version is that the exposed corners do break down, and quite dramatically. That's one of the causes of the dreaded "diamond plate slow-down effect". CBN is known to be worse in that regard than diamond, due to its lower hardness (about 2/3 the hardness when comparing synthetic monocrystalline diamond to synthetic monocrystalline CBN).

They key thing to realize is that "monocrystalline" doesn't imply a single perfect diamond lattice. It just means that the bonds between regions are metallic (crystalline). Synthetic monocrystalline industrial diamonds are both softer and more friable than natural ones, though far less so than resin-bonded polycrystalline ones.

If you look around you can find suppliers like Precision Surfaces and McMaster selling natural diamond powders for lapping super-hard materials like solid carbide. The reason is that economical synthetic diamonds simply aren't up to the task.

So with those details out of the way, what I said stands: Diamond crystals do wear out by breaking down. CBN is worse in that respect than diamond.